8 Phentermine FAQs: How Well It Works for Weight Loss & More - GoodRx (2024)

Newer weight loss medications, such as semaglutide (Wegovy) and tirzepatide (Zepbound), are getting a lot of attention. But there are other options that have been around a lot longer and don’t require an injection.

Phentermine (Adipex-P, Lomaira) is an oral weight loss medication that was FDA approved in 1959. But how does phentermine work? And is it a safe and effective option for weight loss? Let’s explore the answers to these and more common questions about phentermine to help you make informed choices about your weight loss treatment options.

1. Does phentermine work for weight loss?

Yes. Phentermine is FDA approved to help you lose weight. This means it went through clinical studies to show it was safe and effective for weight loss. Phentermine works best if it’s combined with regular exercise and a lower calorie intake.

2. How much weight will you lose while taking phentermine?

How much weight you’ll lose while taking phentermine largely depends on two things: how much excess weight you have and how well you’re able to adjust your diet and exercise habits while taking it.

Losing 3% to 5% of your body weight is considered clinically meaningful. This means it’s likely to have a positive impact on your overall health and well-being.

So how does phentermine measure up? There are several studies that directly address this question:

• A study of over 900 people from Mexico showed that about 80% of people with obesity lost at least 5% of their body weight after 6 months of phentermine treatment.

• One small study from South Korea showed that almost 86% of people taking phentermine for 14 weeks lost 5% or more of their body weight. And more than 50% of people lost 10% or more of their body weight. This study included people considered either overweight or obese.

• Another South Korean study looked specifically at people considered obese (not overweight) who took phentermine for 12 weeks. Almost 96% of study participants lost more than 5% of their body weight. And 62% of the same group lost more than 10% of their body weight.

Phentermine is also an ingredient in the prescription medication Qsymia (phentermine / topiramate). A large study found that 75% of people taking Qsymia over a year lost at least 5% of their body weight, compared to 23% of people taking a placebo (a pill with no medication in it).

3. How does phentermine work for weight loss?

Phentermine works by activating your sympathetic (“fight or flight”) nervous system. So it’s known as a “sympathomimetic” or “stimulant” medication. Stimulants help speed up the functions of your body.

Phentermine seems to lower your appetite. So it can help you eat less calories. This can lead to an energy deficit (meaning you’re burning more calories than you’re eating), which can lead to weight loss. People also report a boost in energy when they’re taking phentermine, which helps them be more active.

4. Is phentermine safe to take?

Yes, phentermine is considered a safe medication. But there are some things to be aware of if you’re taking it.

Phentermine is a controlled substance. This means it carries a risk for misuse or dependence. Studies on phentermine show this isn’t likely to happen when it’s taken as prescribed for obesity. So it’s best to stick to the phentermine dosage your prescriber suggests. Don’t take more than prescribed.

The most common phentermine side effects are dry mouth, trouble sleeping, and headache. In some cases, it may cause tremors, changes in your mood or behavior, and an increased heart rate. Most phentermine side effects will go away as your body adjusts to the medication.

5. Does phentermine have the same side effects as fen-phen?

No. Phentermine was an active ingredient in the medication known as “fen-phen.” This was a combination of two medications: fenfluramine and phentermine. Fenfluramine is another stimulant medication. But fenfluramine and fen-phen weren’t FDA approved for weight loss — they were prescribed off-label.

Unfortunately, the combination of these two medications causes dangerous side effects for some people. This includes damage to heart valves and a fatal lung condition called primary pulmonary hypertension (PPH). Due to these side effects, the FDA recommended removal of fenfluramine (and its “sister” medication, dexfenfluramine) from the market in 1997.

There have been no reported cases of heart valve disease related to phentermine alone. But there have been rare cases of PPH reported with phentermine alone. Still, this side effect is extremely rare and the risk is much lower with phentermine than with fen-phen.

Good to know: Fenfluramine was re-approved by the FDA in 2020 for a certain type of seizure disorder. It now has a required safety program to make sure it’s only prescribed for approved conditions and to lower the risk of heart and lung problems from taking it.

6. Who can take phentermine?

Phentermine is approved if your body mass index (BMI) is higher than 30 kg/m2 or if it’s higher than 27 kg/m2 and you also have other health conditions, such as high blood pressure or diabetes.

But BMI isn’t a perfect tool and may not be an accurate measure of your health. So your medical team may decide to prescribe phentermine even if you don’t fall into one of these approved categories.

It’s helpful to know that phentermine isn’t meant to work on its own. You should incorporate regular exercise and a balanced diet into your routine as well. If you don’t make lifestyle changes along with taking phentermine for weight loss, it may not work as well for you. And you may have a higher chance of gaining weight back.

Who shouldn’t take phentermine?

Phentermine isn’t approved for people under the age of 17. It’s also not recommended if you have certain health conditions, including:

• Heart disease

• Uncontrolled high blood pressure

• Hyperthyroidism (too much thyroid hormone)

• Glaucoma (high eye pressure)

• High levels of nervousness or anxiety

• History of drug misuse

• Pregnancy

• Breastfeeding

• Allergies to other similar stimulant medications

Phentermine may not be the best weight loss medication for you if you consume alcohol regularly. The combination could increase your risk of alcohol-related harm and interfere with your weight loss goals.

And phentermine isn’t recommended if you’re taking monoamine oxidase inhibitors (MAOIs). The combination could cause dangerously high blood pressure.

7. What’s the best time to take phentermine?

Since phentermine can have stimulating effects, it’s best to take it in the morning. An hour or two after breakfast is recommended.

If you take phentermine tablets, your prescriber may have you divide your dose into two smaller daily doses. This can help with afternoon appetite control if needed. And if you take Lomaira (a low-dose phentermine tablet) you’ll take it three times a day, 30 minutes before meals.

Regardless of which form of phentermine you take, you should avoid taking it in the evening. It could interfere with your sleep if you take it too late in the day.

How long does phentermine stay in your system?

A 30 mg dose of phentermine should lower your appetite for about 12 to 14 hours. And it takes about 4 days for your body to clear phentermine after your last dose.

8. How long can you take phentermine?

Phentermine is only approved for short-term use (12 weeks or less). This is because there wasn’t any data that showed it was safe to take long term when it was initially approved. But newer studies show that long-term phentermine use (more than 3 months) is not only safe, but also helps people lose more weight.

Your medical team will help you decide how long to take phentermine based on your medical history and how well it’s working for you.